QuorumCnxManager: use BufferedOutputStream for initial msg

Details

Description

Whilst writing fle-dump (a tool like zk-dump, but to dump FastLeaderElection messages), I noticed that QCM is using DataOutputStream (which doesn't buffer) directly.

So all calls to write() are written immediately to the network, which means simple messaages like two participants exchanging Votes can take a couple RTTs! This is specially terrible for global clusters (i.e.: x-country RTTs).

The solution is to use BufferedOutputStream for the initial negotiation between members of the cluster. Note that there are other places were suboptimal (but not entirely unbuffered) writes to the network still exist. I'll get those in separate tickets.

After using BufferedOutputStream we get only 1 RTT for the initial message, so elections & time for for participants to join a cluster is reduced.